47

Bolting steel structuresBolting has become the most widely used, versatile andreliable method for making field connections in structuralsteel members. The major advantages of bolting overwelding are:

1 Economy, speed and ease of erection 2 Reliability in service 3 Relative simplicity of inspection 4 Fewer and less highly skilled operators required 5 Good performance under fluctuating stresses 6 Ease of making alterations and additions 7 Absence of coating damage 8 No pre-heating of high-strength steels 9 No weld cracking or induced internal stress 10 No lamellar tearing of plates.

Galvanized steel structures

In the construction of galvanized steel structures, boltedconnections offer further advantages. Damage to thegalvanized coating from local heating during welding iseliminated and with it the need for coating repairs to theaffected area.

The high cost of maintenance labour and wide use of steelcommunications towers, exposed industrial structures, steelbridges and power transmission towers, often in remoteareas, have made low maintenance corrosion protectionsystems an essential aspect of design. As a result,galvanizing has become the accepted standard for exposedsteel, placing greater emphasis on bolted joints for structuralsteelwork and leading to development of specialised boltingtechniques.

A wide range of galvanized, sherardised and zinc platedstructural bolts and related fittings is available to meet anysteel construction need.

Zinc coatings for fastenersIn bolted steel structures the bolts and nuts are critical itemson which the integrity of the entire structure depends.

For exterior use these critical fasteners must be adequatelyprotected from corrosion. Where steel members of thestructure are galvanized it is essential that fastenersemployed should also be galvanized or suitably zinc coated

to maintain a uniform level of corrosion protection throughoutthe structure.

Selection of zinc coatings for fasteners

The zinc coating selected is decided primarily by the periodof protection desired which should be equivalent to the life ofthe protective system selected for the structure.

The zinc coating process selected must also produce arelatively uniform coating over small parts of varying shape.With the thicker zinc coatings, allowances in threaddimensions must be made to accommodate the thickness ofthe coating. These requirements dictate that in practice oneof four types of zinc coating will be suitable:

1 Galvanizing2 Zinc plating3 Sherardising4 Mechanical plating

GalvanizingThe galvanizing of fasteners produces a heavy coating of zincideally suitable for long-term outdoor exposure. The coatingis applied by the immersion of clean, prepared steel items inmolten zinc. The resulting zinc coating is metallurgicallybonded to the basis steel, and consists of a succession ofzinc-iron alloy layers and an outer zinc layer.

Fasteners are generally centrifuged immediately onwithdrawal from the molten zinc of the galvanizing bath toremove excess free zinc and produce a smoother finish andcleaner threads.

Australian/New Zealand Standard 4680 ‘Hot dip galvanized(zinc) coatings on fabricated ferrous articles’ provides for a standard minimum coating thickness regardless of fastener dimensions:

Requirements for coating thickness and mass for articles that are centrifuged

Thickness Local Average Averageof articles coating coating coating mass

(all components thickness thickness minimumincluding castings) minimum minimum g/m2

mm µm µm

<8 25 35 250

G8 40 55 390

Notes:1. For requirements for threaded fasteners refer to AS 1214.2. 1 g/m2 coating mass = 0.14 µm coating thickness.

Bolting Galvanized Steel

This chapter gives information on the characteristics, advantages and economics of bolted galvanizedstructures and zinc coated fasteners, and offers comment on bolting procedures when these are influenced bythe presence of zinc coatings. All information given is in accordance with current Australian Standards, andwith the rationalised approach to the design, detailing and fabrication of structural connections developed byAustralian Institute of Steel Construction.

Information given is based on work carried out by Ajax Spurway Fasteners, Australian Institute of SteelConstruction, and International Lead Zinc Research Organization.

48

Oversize tapping allowances for galvanized nuts

To accommodate the relatively thick galvanized coating onexternal threads, it is usual to galvanize bolts of standardthread dimensions, and to tap nuts oversize after galvanizing.AS 1214 ‘Hot dip galvanized coatings on threaded fasteners’specifies the following oversize tapping allowances oninternal threads:

Nominal diameter of internal threads Allowance, mm

Up to M22 0.40 mmM24 0.45 mmM27 0.50 mmM30 0.55 mmM36 0.60 mmM36-48 0.80 mmM48-64 1.0 mm

To ensure that nut stripping strength is adequate afteroversize tapping, galvanized high strength nuts aremanufactured from steel with a higher specified hardnessthan standard high strength nuts, as discussed on page 52.

Galvanized high strength bolts and nuts must be providedwith a supplementary lubricant coating for satisfactory bolttightening. See pages 49 and 51.

Economics of galvanized coatings on boltsCorrosion protection on bolts should match the rest of thestructure and in most circumstances economics favour theuse of galvanized bolts rather than painting after erection.The following table* gives indicative cost-in-placerelationships for unpainted, painted, and galvanized M20bolts in structural applications:

Cost-in-place

Bolt strength grade/ Unpainted Painted GalvanizedBolting procedure

4.6/S 100 190 1108.8/S 120 210 1408.8/T 170 260 190

*T J Hogan and A Firkins, ‘Bolting of steel structures’ AustralianInstitute of Steel Construction

Zinc platingZinc plating on fasteners produces relatively light, uniformcoatings of excellent appearance which are generallyunsuitable for outdoor exposure without additional protection.

There is in general an economic upper limit to the coatingmass which can be applied by plating, although certainspecialised roofing fasteners are provided with zinc platedcoatings up to 35 to 40 µm thick. Where heavy coatings arerequired galvanizing is usually a more economic alternative.

Zinc plated bolts having a tensile strength above 1000 MPamust be baked for the relief of hydrogen embrittlement.

Zinc plated high strength bolts and nuts must be providedwith a supplementary lubricant coating to provide forsatisfactory bolt tightening as discussed on pages 49 and 51.

Australian standards for zinc plating require that one of arange of chromate conversion coatings be applied inaccordance with Australian Standard 1791 ‘Chromateconversion coatings on zinc and cadmium electrodeposits’.Clear, bleached, iridescent or opaque films may be produced, depending on the level of resistance to wetstorage staining required.

Australian Standard 1897 ‘Electroplated coatings onthreaded components (metric coarse series)’ specifies platingthicknesses which can be accommodated on externalthreads to required tolerances.

SherardisingSherardising produces a matt grey zinc-iron alloy coating.The process impregnates steel surfaces with zinc by rumblingsmall components and zinc powder in drums heated to atemperature of about 370°C. The least known of the variousprocesses for zinc coating steel, sherardising is not used inAustralia. The process is characterised by its ability toproduce a very uniform coating on small articles.

The thickness of sherardised coatings is generally of theorder of 15 µm but can vary depending on cycle time from7.5 to 30 µm. Sherardised coatings therefore fall betweenzinc plated and galvanized coatings in thickness and life.

Although sherardising is an impregnation process there issome build up in dimensions. British Standard 729 ‘Zinccoatings on iron and steel articles, Part 2: Sherardisedcoatings’ recommends an oversize tapping allowance of 0.25mm on nuts to ensure easy assembly with sherardised bolts.

Mechanical (peen) platingMechanical or peen plating offers advantages in the zinccoating of fasteners. Coatings are uniform, and because theprocess is electroless there is no possibility of hydrogenembrittlement. High strength fasteners not susceptible toembrittlement need not be baked after coating. Lubricantcoatings must be applied to ensure satisfactory tightening.

Influence of the galvanizedcoating on design The presence of either galvanized coatings or zinc plating onhigh strength bolts, and galvanized coatings on structuralmembers may need to be taken into account in design. Thefollowing factors should be considered:

1 Slip factors of mating surfaces2 Fatigue behaviour of bolted galvanized joints3 Bolt relaxation 4 Effect of galvanized coating on nut stripping strength5 Torque/induced tension relation in bolt tightening

1. Slip factors affecting mating surfacesIn a friction type bolted joint all loads in the plane of the jointare transferred by the friction developed between the matingsurfaces. The load which can be transmitted by a frictiontype joint is dependant on the clamping force applied by thebolts and the slip factor of the mating surfaces.

Australian Standard 4100 ‘Steel structures’ assumes a slipfactor of 0.35 for clean as-rolled steel surfaces with tight millscale free from oil, paint, marking inks and other appliedfinishes. AS 4100 permits the use of applied finishes such asgalvanizing in friction type joints, but requires that the slipfactor used in design calculations be based on test evidencein accordance with the procedures specified in Appendix J ofthe standard. Tests on at least three specimens are required,but five is preferred as the practical minimum.

Bearing type joints are not affected by the presence ofapplied coatings on joint faces, so galvanizing may be usedwithout affecting design strength considerations.

Slip factors of galvanized coatings

Research conducted in Australia and overseas shows meanslip factors for conventional galvanized coatings over a largenumber of tests to be in the range 0.14 to 0.19, ascompared to 0.35 for clean as-rolled steel.

Design values take these lower slip factors into account, andgalvanized steel is used widely in high strength friction type joints.

49

Work by Professor WH Munse* for International Lead ZincResearch Organisation, and others, shows that the slipfactors of galvanized surfaces can be substantially improvedby treatments such as wire brushing, light •brush off• gritblasting, and disc abrading. In each case the treatment mustbe controlled to provide the requisite scoring or rougheningto expose the alloy layers of the coating. Care must be takento ensure that excessive coating thickness is not removed.

The following table s hows the res ults of s lip factor tes tingvarious galvanized s urfaces in four-bolt joints .

No ofS urface treatment tes ts Average Min Max

As received 15 0.14 0.11 0.18Weathered 3 0.20 0.15 0.26Wire brus hed 4 0.31 0.27 0.33G rit blas ted 2 0.31 0.28 0.34

* WH Muns e •High s trength bolting of galvanized connections •pres ented to the s ympos ium •B olting galvanized connections andnew s teel des ign s pecifications •, Aus tralian Ins titute of S teelC ons truction and Aus tralian Zinc Development As s ociation, 1968

It is important to recognise more recent developments ingalvanizing technology which produce harder final layers ofzinc. Testing has been undertaken to establish higher slipfactors for structural steel produced in the moderngalvanizing facilities. Designers should check with thegalvanizer before assuming a slip factor for slip critical jointsin a structure.

Fully alloyed •grey• galvanized coatings which can result fromthe galvanizing of silicon steels have also been shown todevelop higher slip factors.

Slip factors given here are indicative only, and designs mustbe based on proven slip factors established by testing inaccordance with the requirements of AS4100, Appendix J.

2. Fatigue behaviour of bolted galvanized jointsWhile the galvanized coating behaves initially as a lubricant ithas been shown in fatigue tests carried out by Munse thatafter the first few cycles galvanized mating surfaces tend to•lock up• and further slip under alternating stress is negligible.The figure below taken from work by Munse illustrates thiseffect. Note the rapidly decreasing amplitude of slip from firstto second and then to fifth stress cycle.

Further indications of •lock up• behaviour became apparentwhen joints were disassembled, galling of the galvanizedcoating being observed in regions where there had been highcontact pressure. Where no initial slip can be tolerated areduced slip factor must be used in design. The slip factor ofthe galvanized coating may be improved by wire brushing or•brush off• grit blasting as discussed above, but slip factorsfor galvanized surfaces post treated in this way must beverified in accordance with Appendix J of AS 4100.

3. Bolt relaxationThe possible effect of bolt relaxation caused by the relativelysoft outer zinc layer of the galvanized coating on the membermust also be considered. If the zinc coating flowed under thehigh clamping pressure it could allow loss of bolt extensionand hence tension. This factor was also studied by Munse.He found a loss of bolt load of 6.5 percent for galvanizedplates and bolts due to relaxation, as against 2.5 percent foruncoated bolts and members. This loss of bolt load occurredin 5 days and little further loss is recorded. This loss can beallowed for in design and is readily accommodated.

S tres s vers us s lip for fatigue s pecimen s ubjected to alternatings tres s of 23.2 kips /in2 G alvanized members and bolts . F irs t, s econdand fifth s tres s cycles . W H Muns e •S tructural B ehaviour of hotgalvanized bolted connections •. Proceedings 6th InternationalC onference on Hot Dip G alvanizing 1968.

4. Effect of galvanized coatings on nutstripping strengthGalvanizing affects bolt-nut assembly strength primarilybecause the nut must be tapped oversize to accommodatethe thickness of the zinc coating on the bolt thread. Theoversize tapped thread reduces the stripping strength of thenut when tested on a standard size threaded mandrel.

In high strength bolting correct tightening is essential andAustralian Standard 1252 •High strength steel bolts withassociated nuts and washers• makes no exceptions foroversized tapped galvanized nuts and specifies that all highstrength nuts must meet the full stripping load when testedon a standard-size hardened mandrel. To meet thisrequirement galvanized high strength nuts must have a higherspecified hardness in accordance with AS 1252. For thisreason normal high strength nuts must not be galvanized andtapped oversize for use in high strength bolted joints.

5. Torque/induced tension relation intighteningThe relationship between torque and induced tension intightening is dependent on bolt and nut thread surface finish,thread surface coatings, and conditions of lubrication.

Galvanized coatings and zinc plated coatings on threadsboth increase friction between the bolt and nut threads, and make the torque/induced tension relation much more variable.

The effect of lubricants on galvanized or zinc plated threadsis significant. The torque/tension relationship shows muchreduced variability, and it becomes possible to tighten inexcess of the minimum tension without danger of boltfracture.

Torque/induced tens ion-relation for M20 high s trength s tructuralbolts , galvanized and lubricant coated, and as -galvanized.

AS4100

51

The diagram shows the torque/induced tension relation foras-galvanized, and lubricant coated galvanized M20 highstrength structural bolts. With the as-galvanized assembliesthere is a wide scatter in induced tension at any one torquelevel and torque could not be used to provide a reliablemethod for gauging the required minimum bolt tensionspecified in AS 4100 before bolt fracture occurred. Boltfailures in torsion resulted from the high friction between theas-galvanized bolt and nut threads. Accordingly, AS4100does not recognise the use of the torque control method fortensioning galvanized or zinc plated bolts, as discussedunder ‘Full tightening’ page 56.

Lubricant coatings on threads

Because of the poor torque/induced tension relationship ofgalvanized or zinc plated high strength bolt/nut assembliesAS 1252 specifies that supplementary lubrication must beprovided. Lubricants should be pre-applied by themanufacturer.

Effectiveness of lubricants is checked by an assembly testwhich requires the bolt to withstand a minimum of between180° and 420° from a snug position, depending on boltlength, before bolt fracture occurs.

Even when lubricant coated, galvanized and zinc plated highstrength bolt/nut assemblies produce a wide scatter ininduced tension for a given level of torque during tightening.Therefore only part-turn tightening or direct tension indicatortightening methods may be used as discussed on page 56.

Structural bolts and boltingtechniquesThree main types of metric bolt are used in structuralengineering in Australia:

• Commercial bolts to AS1111, strength grade 4.6• Medium strength or tower bolts to AS1559, strength

grade 5.6• High strength structural bolts to AS 1252, strength

grade 8.8

Design provisions for structural bolts are contained inAustralian Standard 4100-1998 ‘Steel structures’. Thisstandard, in limit states design format, supersedes AS 1250which was in a working stress format. AS 4100 alsoincorporates the design and installation clauses of highstrength bolts from AS1511 which it also supersedes.

Relevant Australian Standards

Relevant material standards referenced by AustralianStandard 4100 are the current editions of:

AS1110 ‘ISO metric hexagon precision bolts and screws’

AS1111 ‘ISO metric hexagon commercial bolts and screws’

AS 1112 ‘ISO metric hexagon nuts, including thin nuts,slotted nuts and castle nuts.’

AS 1252 ‘High strength steel bolts with associated nuts andwashers for structural engineering’

AS 1275 ‘Metric screw threads for fasteners’

AS 1559 ‘Fasteners – bolts, nuts and washers for towerconstruction’

Strength designations, metric boltsThe strength of metric structural bolts is specified in terms ofthe tensile strength of the threaded fastener and definedaccording to the ISO strength grade system which consistsof two numbers separated by a point, for example 4.6. Thefirst number of the designation represents one hundredth of

the nominal tensile strength (MPa) and the number followingthe point represents the ratio between nominal yield stressand nominal tensile strength.

For example a Property Class 4.6 bolt has:

Tensile strength of 4 x 100 = 400 MPaYield stress of 0.6 x 400 = 240 MPa

Galvanized commercialgrade boltsMetric commercial grade low carbon steel bolts used in thestructural steel industry are manufactured to AustralianStandard 1111 ‘ISO metric hexagon commercial bolts andscrews’ which calls for a tensile strength of 400 MPaminimum, with the Property Class designation 4.6. Designstresses are specified in AS 4100.

Identification of commercial bolts

Commercial bolts Property Class 4.6 carry the maker’s nameand the metric M on the bolt head. Nuts generally aresupplied to Strength Grade 5 and carry no markings.

Galvanized tower bolts Transmission towers are designed as critically stressedstructures and the very large number of towers usedprovided the incentive to reduce weight and cost byapplication of the plastic theory basis for design. This designconcept calls for a higher strength bolt than the standardcommercial 4.6 bolt. The medium strength tower bolt toAustralian Standard 1559 ‘Fasteners – bolts, nuts andwashers for tower construction’ was developed to meet thisneed. Property Class is 5.6 and galvanizing is the standardfinish to provide corrosion protection matched to thestructure.

As maximum shear strength values are required the thread iskept out of the shear plane. Transmission towers are oftenerected in high snow country and it is also necessary tohave a bolt with good low temperature notch toughness.Short thread lengths and specified notch ductility meet theserequirements.

AS 1559 calls for the following properties:

Tensile strength minimum 480 MPaYield stress, minimum 340 MPaStress under proof load 320 MPaCharpy V-notch impact at 0°C:

Average of 3 tests, minimum 27JIndividual test, minimum 20J

Nut locking for tower bolts

Transmission towers are constructed from galvanizedstructural sections using single bolted joints, and positiveprevention of nut loosening is necessary in critical situations.This requirement is met by effective initial tightening andsome additional measure to ensure nut locking, such aspunching and distortion of the bolt thread at the outer nutface after tightening, or the use of galvanized prevailingtorque type lock nuts.

Identification of tower bolts

Galvanized metric tower bolts carry the metric M on the bolthead together with the letter T for Tower, and the maker’sname. Property Class 5 nuts are normally used, withoutmarkings.

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Galvanized high strengthstructural boltsThe use of high strength structural bolts to AS 1252 inappropriate structural designs provides improved economyand efficiency in the fabrication of galvanized structures bypermitting:

1 Smaller bolts of higher strength2 Fewer bolts and bolt holes, resulting in:3 Lower fabrication cost for members4 Faster erection and reduced erection cost

AS 1252 calls for the following properties:

Tensile strength, minimum 830 MPaYield stress, minimum 660 MPaStress under proof load 600 MPaMinimum breaking load:

M20 nominal diameter 203 kNM24 nominal diameter 293 kN

In structural applications galvanized high strength structuralbolts are commonly used in M20 and M24 metric diameter inboth flexible and rigid connections. M30 diameter is lessused in structural applications, particularly when fulltightening is required to AS 4100, because of the difficulty ofon-site tensioning to achieve specified minimum bolttensions. M36 should never be specified if full tensioning toAS 4100 is required.

Galvanized high strength nuts

Nut threads are tapped oversize after galvanizing to allow forthe increased thread diameter of the galvanized bolt. Toensure that nut stripping strength is adequate after oversizetapping, galvanized high strength nuts are manufacturedfrom steel with a higher specified hardness than other highstrength nuts, as discussed on page 48.

AS 1252 specifies the following mechanical properties:

Rockwell VickersProof hardness hardnessload HRC HRB HV

Nut stresstype MPa Max Min Min Max Min

Galvanized 1165 36 24 – 353 260All others 1075 36 – 89 353 188

Identification of high strength bolts, nuts andwashers

Galvanized high strength bolts to AS 1252 Property Class8.8 can be identified by three radial lines on the bolt head,with the maker’s name and the metric M. Nuts to PropertyClass 8 for use with structural bolts can be identified bythree circumferential lines on the face of the nut. Relative tonominal thread size, high strength structural bolt heads andnuts are visibly larger than commercial bolts and nuts. Flatround washers for use with high strength structural bolts canbe identified by three circumferential nibs.

Modes of force transfer inbolted jointsIn the design of individual bolts in bolted structuralconnections, there are three fundamental modes of forcetransfer:

1. Shear/bearing mode. Forces are perpendicular to thebolt axis and are transferred by shear and bearing on theconnecting plies – bolting categories 4.6/S, 8.8/S and8.8/TB described below.

2. Friction mode. Forces to be transferred areperpendicular to the bolt axis as in shear/bearing mode,but load carrying depends on the frictional resistance ofmating surfaces – bolting category 8.8/TF

3. Axial tension mode. Forces to be transferred areparallel to the bolt axis – may apply in combination withother bolting categories.

Bolting category systemThe following bolting category identification system is basedon that used in AS4100:

Category 4.6/S – Commercial bolts used snug tightCategory 8.8/S – High strength structural bolts used

snug tightCategory 8.8/TF – High strength structural bolts fully

tightened in friction type jointsCategory 8.8/TB – High strength structural bolts fully

tightened in bearing type joints.

This category designation system is derived from theStrength Grade designation of the bolt, for example 8.8, andthe bolting design procedure which is based on thefollowing supplementary letters:

S represents snug tightTF represents fully tensioned, friction type jointTB represents fully tensioned, bearing type joint

Category 4.6/S refers to commercial bolts of StrengthGrade 4.6 tightened snug tight as described undertightening procedures, page 56. (Snug tight is the final modeof tightening for bolting categories 4.6/S and 8.8/S, and thefirst step in full tensioning for bolting categories 8.8TF and8.8TB).

Category 8.8/S refers to high strength structural bolts ofStrength Grade 8.8 used snug tight.

High strength structural bolts in the snug tight condition maybe used in flexible joints where their extra capacity can make

External load External load resistedby shear of bolt and

bearing on connected plies

External load

Hardened washer Frictional forces atmating surfaces resist external load

Clearance holes in plates

Heavy nut

Bolt pre-loadproduces friction

Clamping force developed by tightening of nut

External load

Bolt reactionor pre-load

External load

53

them more economic than commercial bolts. The level oftightening achieved is adequate for joint designs wheredeveloped bolt tension is not significant. Behaviour of the boltunder applied loads is well known and accepted.

Category 8.8T refers to both categories 8.8 TF and 8.8/TB

Category 8.8/TF refers to high strength structural boltsStrength Grade 8.8 used in friction type joints, fully tensionedin a controlled manner to the requirements of AS 4100.

AS 4100 specifies that friction type joints must be usedwhere no slip is acceptable. They should also be used inapplications where joints are subject to severe stressreversals or fluctuations as in dynamically loaded structuressuch as bridges, except in special circumstances asdetermined by the engineer. Where the choice is optional,bearing type joints are more economic than friction type.

Category 8.8/TB refers to high strength structural boltsStrength Grade 8.8 used in bearing type joints, fullytensioned in a controlled manner to the requirements ofAS4100.

Variation in design values with boltstrength and joint designDesign values vary with joint design, bolt type and level ofbolt tightening. The table below indicates the range ofdesign values in shear which apply to bolts of the samenominal diameter (M20) in varying strength grades, used invarious joint designs, in standard size holes (Kh=1), inaccordance with AS4100.

Design value in shear, kN

Bolt and Threads Threadsjoint included in excluded fromdesignation shear plane shear plane

4.6/S 44.6 62.3

8.8/S 92.6 129

8.8/TF 35.5 35.5

8.8/TB 92.6 129

*Slip factor = 0.35

Design for bolted structuraljointsA summary of structural design procedures to AS 4100 hasbeen produced by Arun Syam of Australian Institute of SteelConstruction and Arthur Firkins, Consultant, and published by Ajax Spurway Fasteners in their Fasteners Handbook’,pages 54 to 68.

Copies are available from AISC and Ajax Spurway Fasteners.

Design for high strengthboltingAS 4100 specifies conditions for the application of highstrength structural bolts in both friction type and bearing typejoints. Bolts are tightened to the same minimum inducedtension in both types of joint.

Tension type jointsFor joints in which the only force is an applied tensile force inthe direction of the bolt axes, the tensile force on any boltshould not exceed the following table.

Nominal diameterMaximum permissible bolt tension, kN

of bolt, mm Static load

M16M20M24M30

Minimum MinimumBolt tensile yield

Bolting strength strength strength Australiancategory grade (MPa) (MPa) Name Standard Method of tensioning/remarks

4.6/S 4.6 400 240 Commercial AS 1111 Use snug tight.Least costly and most commonlyavailable 4.6 grade bolt.

8.8/S 8.8 830 660 High strength AS 1252 Bolts used are snug tight.structural The high strength structural bolt

has a large bolt head and nutbecause it is designed to withstandfull tensioning.It can also be used in a snug tightcondition.

8.8/TF 8.8 830 660 High strength AS 1252structural bolt,fully tensioned For categories 8.8/TF and 8.8/TB boltsfriction type joint are fully tensioned to the requirements

of AS 4100. Cost of tensioning is an

8.8/TB 8.8 830 660 High strength AS 1252important consideration in the use

structural bolt,of these bolting categories.

fully tensionedbearing type joint

Bolt types and bolting categories

104163234373

56

Tightening procedures forhigh strength structural boltsThe installation and tightening of a high strength structuralbolt/nut assembly is at least as costly as the bolt/nutassembly itself, and the selection of bolt type and bolttightening procedure is an important consideration in theeconomics of high strength bolted structures.

Snug tightening

Snug tight is defined in AS 4100 as the full effort of a man ona standard podger spanner, or the point at which there is achange in note or speed of rotation when a pneumaticimpact wrench begins impacting solidly. Podger spanners aregraded in length in relation to bolt size and strength, and are,for example, of the order of 450 mm long for M20 highstrength structural bolts, and 600 mm long for M24 highstrength structural bolts.

Snug tightening is applied in the following situations:

1 The final level of bolt tightening in general structuralbolting using commercial bolts – Category 4.6/S.

2 A final level of bolt tightening using high strength structuralbolts – Category 8.8/S. Different design values must beapplied than for procedures 8.8/TF and 8.8/TB using thesame bolts, as discussed on page 52.

3 An intermediate level of bolt tension applied as the firststage in full tightening – Categories 8.8/TF and 8.8/TB.

The growing popularity of high strength structural bolts to AS1252 used in a snug tight condition leads to the situationwhere bolts may require full tightening to AS 4100 in oneapplication and only snug tightening in another. To preventconfusion and ensure correct tightening the designermust indicate clearly the level of tightening required, inboth drawings and specifications. Steps must be taken toensure that this information is conveyed to all those involvedin installation, tightening, and inspection.

Snug tighteningWhen snug tightening is used as the first stage for fulltightening in procedures 8.8/TF and 8.8/TB, the intention isto bring the plies into ‘snug’ contact ready for final tightening.The clamping force applied by snug tightening is highlyvariable as illustrated below, but is not significant when boltsare subsequently fully tightened – since the bolt tension/boltelongation curve is relatively flat, variations in the snug tightcondition result in only small variations in final bolt tension.

Full tightening (minimum bolt tension)For joints designed in accordance with AS 4100, either as8.8/TF friction type or 8.8/TB bearing type, bolts must befully tightened to the following minimum tensions:

Nominal bolt diameter Minimum bolt tension, kN

M16 95M20 145M24 210M30 335M36* 490

*If M36 bolts are specified the part turn method of tightening shouldbe used only after special investigation into the capacity of theavailable equipment.

To attain these bolt tensions AS 4100 permits galvanized orzinc plated bolts to be tightened by either the part turn of nutmethod, or by the direct tension indicator method. Torquecontrol tightening of galvanized or zinc plated bolts and nutsis prohibited in AS 4100 because of the variabletorque/induced tension relationship of zinc coatings evenwhen lubricant coated.

Nut rotation from the snug-tight condition AS 4100

Disposition of outer face of bolted partsNotes 1, 2, 3 and 4

Bolt length(underside Both faces One face normalof head to normal to to bolt axis and Both facesend of bolt) bolt axis other sloped sloped

Up to andincluding 1/3 turn 1/2 turn 2/3 turn4 diameters

Over 4 dia-meters but 1/2 turn 2/3 turn 5/6 turnnot exceeding8 diameters

Over 8 dia-meters but 2/3 turn 5/6 turn 1 turnnot exceeding12 diameters(see Note 5)

1 Tolerance on rotation: for 1/2 turn or less, one-twelfth of a turn(30°) over and nil under tolerance; for 2/3 turn or more, one-eighth of a turn (45°) over and nil under tolerance.

2 The bolt tension achieved with the amount of nut rotationspecified above will be at least equal to the specified minimumbolt tension.

3 Nut rotation is the rotation relative to the bolt, regardless of thecomponent turned.

4 Nut rotations specified are only applicable to connections inwhich all material within the grip of the bolt is steel.

5 No research has been performed to establish the turn-of-nutprocedure for bolt lengths exceeding 12 diameters. Therefore,the required rotation should be determined by actual test in asuitable tension measuring device which simulates conditions ofsolidly fitted steel.

Part turn tightening1 Line up holes with drift pins to maintain dimensions and

plumbness of the structure. 2 Fit bolts in remaining holes. Use taper washers if surface

slope exceeds 3° and use flat washers under the rotatingcomponent.

3 Tighten all bolts to snug tight position, progressingsystematically from the most rigid part of the joint to thefree edges.

4 On large joints take a second run to check all bolts aresnug tight.

Bolt tension/boltelongation curvefor a typical highstrength structuralbolt.

The range of finalbolt tentions afterpart turn tighteningexceeds minimumspecified bolttension despitevariability in snugtightening.

57

5 Match mark installed nuts and bolts using a punch toshow that snug tightening is complete. These marks canthen be used for final tightening and inspection.

6 Complete tightening using the part turn method accordingto the table on page 56. Tightening should proceedsystematically from the most rigid part of the joint to itsfree edges. Wrench sockets should be marked atpositions 180° apart to guide the operator in tightening.

7 Knock out drift pins, replace with bolts. 8 Bring these bolts to snug tight position as in step 3,

match mark as in step 5 and complete tightening as instep 6.

9 Mark joint to indicate tightening has been completed. Onemethod is to draw lines with crayon between each bolthead forming a squared pattern.

Direct tension indicator tighteningSeveral direct tension indicating devices have beendeveloped to provide a simple method of checking thatminimum bolt tension has been developed. The mostcommonly used in Australia is the load indicator washer.

The load indicator is similar in size to a normal circularwasher, with four to seven protrusions depending on size, on one face. It is assembled under the bolt head so that the protrusions bear on the underside of the head. As thebolt is tightened the protrusions are flattened, and reductionof the gap by a specified amount indicates that minimum bolt tension has been reached. For use with galvanizedstructural bolts load indicator washers are supplied with a galvanized finish.

Load indicating washers and sketch showing washer fitted under bolt head. Note gap which is reduced as nut is tightened.

Tightening procedure with load indicator washers

1 Ensure that the bolts are high strength bolts to AS 1252. 2 Place load indicator on the bolt with protrusions abutting

the underside of the bolt head or abutting a structural flatwasher if the bolt head is to be turned in tightening.

3 Fit the bolt into place and assemble with nut and standard

hardened washer. If a taper washer is required it ispreferable that this be fitted under the nut but alternativelyit may be placed between the load indicator and thestructural steel.

4 Carry out a preliminary tightening to snug tight position,using a podger spanner or pneumatic impact wrench. It isimportant to begin tightening at the most rigid part of thejoint progressing systematically to the free edges. On largejoints take a second run over bolts to check that all aresnug tight.

5 Carry out final tightening by reducing the gap betweenbolt head and load indicator to approximately 0.25 mm forgalvanized bolts. In aggressive exposure conditions thegap may be fully closed to exclude moisture.

Should a nut be slackened after being fully tightened a newload indicator must be fitted before the second tightening.

Fitting load indicator under nut

In applications where it is necessary to rotate the bolt headrather than the nut, the load indicator can be fitted under nutusing a special nut face washer which is heat treated to thesame hardness as the bolt. Care must be taken that the nutface washer is fitted concentric with the nut and the correctway up, otherwise it may turn relative to the load indicatorresulting in inaccurate load indication due to damage to theprotrusions.

Experience has shown that on medium to large projects the extra cost of load indicators is offset by major savings in installation, supervision, and inspection of high strength joints.

Inspection of high strengthbolted jointsBecause of the increasing use of high strength structuralbolts in the snug tight condition the designer must clearlyindicate the level of tightening required in drawings andspecifications, and he must ensure that this information isconveyed to all those involved in installation, including theinspector.

In structural joints using either 4.6/S or 8.8/S procedures thesite inspector need only be concerned that the correct bolttype and number of bolts have been used in the joint. Sincethe level of tightening required is snug tight, this would havebeen achieved during erection.

In joints using galvanized bolts and 8.8/TF or 8.8/TBprocedures, only visual inspection is necessary. The inspectorshould check that the correct fasteners and washers havebeen used and correctly installed, and that none showphysical damage which might indicate they been driven intomis-aligned holes.

Galvanized bolts which have been tightened by the part turnof nut method can be checked by their match markings.Where load indicating washers have been used for finaltightening, inspection is greatly simplified.

Tightening of bolts by the torque control method has beendeleted from AS 4100. For guidance on the use of a torquewrench for inspection refer to AS 4100 Supplement 1-1990,Appendix CK.

Part turn tightening.When tightening byhand or for permanentindication of tighteningbolts and nuts shouldbe match marked. Before final tightening After

58

Flush spliced structuraljoints in galvanized steelThe increasing popularity and use of hot dip galvanizing as anarchitectural finish for structural steel members has sometimesbeen limited by the need to make structural connections.Welding, while practical, requires coating touch up which mayspoil the visual continuity of the galvanized coating in someapplications. Conventional bolted connections, while versatileand economic, can be visually obtrusive.

A new method of flush-splicing structural steel members con-ceived by Arthur Firkins, formerly Director of Technical Services,Australian Institute of Steel Construction, has been developedunder the auspices of Galvanizers Association of Australia.

The new connection uses flat-head countersunk Unbrakohigh strength socket screws through beam flanges intothreaded holes in the flange and web connecting members.The result is a flush finish to beam flange surfaces withoutprotruding bolt heads or nuts, in a joint with the performancecharacteristics needed in structural applications.

Structural performance

In order to investigate joint behaviour, a test specimen wassubjected to tensile testing at the University of Sydney todetermine the flange force transfer capacity of a typicalsplice. Test results showed that the splice conformed to therequirements of Australian Standard 4100 ‘Steel Structures’.The test results also confirmed the designed capacity of theflange beam calculated in accordance with AS4100.

As a result of this testing, structural engineers can nowincorporate unobtrusive flush-spliced structural connections,confident that their design will meet the requirements of AS4100.

Fasteners and threads

The fasteners employed are Unbrako high strength flat-headsocket screws, ISO metric series, mechanically zinc plated to acoating thickness of 25µ to give adequate corrosion protection.

The specification for these bolts is:Material: Unbrako high-grade alloy steel*.Hardness: Re36-44Ultimate tensile strength: 1100MPa0.2% yield stress: 990MPaThread class: 4g.

* In the International method of designating bolt strength these boltswould be classified as Grade 10.9.

M12, M16 and M20 screw sizes are used.

Design of flush-bolted splices

Dimensional criteria for connections in commonly usedbeams are given in the table below. These criteria apply toboth fully-bolted splices (Drawing A) and bolted/weldedsplices (Drawing B). This system will allow relatively largeflange force transfer in members of all types and sizes. Spliceplates should be at least equal to flange or web thicknessand not less than screw diameter.

Installation procedures

Procedures for the installation of Unbrako socket headscrews is contained in the product manual published byUnbrako.

70 70 45

45 45

45 70

70

70

70

(A) Fully bolted splice (B) Bolted/welded splice (alternative)

‘n’ rows @ 70 pitch - as per design equal

weld

equal

weld

40 40 3535 40 40 3535

Dimensional criteria

Flange Web

Size Flange Web Width Thick Width Thick Size Sizetf tw mm mm mm mm

UB Sections

200 UB 30 9.6 6.3 50 20 150 6 M16 M12250 UB 37 10.9 6.4 50 20 150 6 M16 M12310 UB 40 10.2 6.1 50 20 150 6 M16 M12360 UB 51 11.5 7.3 50 20 150 6 M16 M12410 UB 54 10.9 7.6 50 20 150 8 M16 M12460 UB 67 12.7 8.5 50 20 150 8 M20 M16530 UB 82 13.2 9.6 75 20 150 10 M20 M16

UC Sections

250 UC 73 14.2 8.6 100 20 150 8 M20 M16200 UC 46 11.0 7.3 75 20 150 8 M20 M12

* Unbrako flat head socket screws Grade 10.9

1 Suggested criteria in the table should be verified for specific design load cases.2 For serviceability state, "Ply in bearing (beam flange) will usually govern design" (AS4100 9.3.2.4.(2)).3 Ultimate failure in the test was the flange plate component failing in tension.4 Flange plate component thickness should be greater than flange thickness and equal to or greater than bolt diameter.5 Web plate component thickness should be greater than web thickness.6 "n" = number of rows of bolts in flange or web as required by design – see Drawing (A). Note: Bolt shear strength (10.9) will rarely govern.7 Bolts should be specified as "Unbrako flat-head socket screws Grade 10.9, mechanically zinc/tin plated to a coating thickness of 25µ".8 Holes in flange plates should be tapped 0.1mm oversize to allow for the coating thickness on screw threads. 9 Tapped threads should be plugged during the galvanizing process using bolts of appropriate diameter (Grade 4.6 hex head uncoated).

Fully-bolted splice,drawing A, andbolted/welded splice,drawing B.

Member Flange plates Web plates Bolts*